Audiences for content blocks, such as broadcast media or online streamed media, including commercial advertising, are typically measured via single-source data panels consisting of individuals or households recording content exposure over time. Potential audience values for each block are then used for planning and purchasing advertising slots. By utilizing a single-source, changes in behavior can be measured over time responsive to different campaigns. In non-broadcast platforms, such as online media delivery systems, content publishers may insert ads for users, via interstitial ads in videos, banners, etc. Panel members may run monitoring software such as browser plug-ins or extensions that transmit identifiers of received content to the panel provider for aggregation. However, aggregating data from a large variety of sources may result in a significant lack of privacy for panel members.